1. Field Of The Invention
The present invention relates to a subsea valve actuator comprising an override shaft reaching from inside a fluid filled and pressure compensated actuator housing to the exterior thereof via a bearing and sealing arrangement, wherein a valve position indicator is arranged visible on the exterior of the actuator housing.
2. Description Of The Related Art
Basically, valves for subsea production fluids operate at great depths and corresponding high pressure, low temperature and in conditions of poor visibility. An accurate operation of the valve is crucial and failure may result in loss of production, environmental pollution or safety hazards. A subsea valve is typically remotely controlled through valve actuators that are hydraulically or electrically driven for shifting the valve between open, closed and any intermediate positions. The position of the valve may be monitored remotely by means of a position sensor arranged in the actuator and connected to a control centre at a host facility. In a case of failure in the valve positioning system or in the valve position detection function, subsea valve actuators are often arranged for manual shifting of the valve by overriding the actuator's normal drive system in response to a visual inspection of the valve position, both of which are accomplished on site by divers or by remotely operated vehicles (ROV).
Subsea valve actuators having manual override and visual position indicator means are previously known. Examples are found in EP 0 384 607 A1, US 2003/0116200 A1, U.S. Pat. No. 6,487,960 B1, U.S. Pat. No. 4,213,480 A, e.g.
In a prior art valve actuator, the manual override comprises an override shaft which engages the drive system and which reaches out from a pressure compensated actuator housing with a shaft end that is accessible for manual shifting of the valve. An indicator for the valve position is likewise accessible for visual inspection from outside the actuator housing. The visual indicator comprises an indicator needle attached to a shaft that reaches into the actuator housing and is connected to the drive system via pinion and reduction gears. Besides a mechanically complex structure to ensure synchronization between the movement of the valve and the corresponding reading of the indicator, each penetration of the actuator housing involves a potential risk for ingress of sea water, or the risk of bleeding from the actuator housing which is typically filled with oil or grease.